This invention relates, in general, to microwave signal processing and, more specifically, to magnetostatic-wave devices used for filtering, delaying, and enhancing microwave signals.
One of the traditional methods of processing microwave frequency signals required the use of mixers or converters to lower the signal frequency to a more workable range. At this lower range, delay lines, transversal filters, and signal-to-noise enhancers could be constructed according to surface-accoustic-wave (SAW) techniques. It is now possible, however, to process the microwave frequency information directly without converting to a lower frequency by using magnetostatic-wave (MSW) devices which are capable of performing the processing functions at the speed required for operation at microwave frequencies.
Compared with SAW technology, MSW technology offers lower propagating losses, simpler transducers which do not require narrow line width photolithography, and broader frequency tunability by means of an externally applied magnetic field. In addition, when signals are processed at microwave frequencies, the signals are processed in near real time, thereby allowing faster decision making capability than with previous techniques. The elimination of the mixer also reduces any errors introduced into the receiving system by distortion caused by the mixer.
Although magnetostatic-wave devices offer certain advantages over conventional technology for processing microwave signals, MSW devices still present certain limitations and problems to their users in MSW filter applications. One of these limitations involves the bandpass shape characteristics of the MSW filter. Every MSW filter has a transducer located at each end of the ferrimagnetic material used in the MSW filter. It has been determined that the passband of the MSW filter is determined mostly by the response of the transducers. In order to improve the passband characteristics, multi-fingered transducers have become popular and offer passband performance exceeding that of MSW filters using single conductor transducers. The multi-fingered transducer behaves very much like an axis phased array, with the individual fingers providing the element factor, and the spatial array of fingers providing the array factor. The element factor, for realizable finger widths on the order of 1 to 2 mils, has a bandwidth of 1 to 2 GHz. Thus, the array factor provides an important contribution to useful filter response.
The Fourier transform of the array factor determines the filter passband characteristics by either variations in finger spacing or by tapering the individual finger element amplitudes which act upon the ferrimagnetic material of the MSW filter. For mid to high microwave frequencies, spacing variations require increased high resolution photolithography, so in many cases it is desirable to resort to amplitude tapering as the means for determining the array factor of the transducer. Amplitude tapering, or weighting, has traditionally been done by varying the finger length. U.S. Pat. No. 4,314,214, issued on Feb. 2, 1982, discloses, in FIG. 8, a magnetostatic-wave device using a multi-fingered transducer with amplitude weighting created by the different finger lengths of the transducer. Amplitude weighting is also discussed briefly in the article "Magnetostatic-Wave Devices Move Microwave Design Into Gigahertz Realm," Electronics, May 8, 1980, page 127, column 2, where it is stated that a few of the parameters available for amplitude weighting are the finger width, length, and distance from the YIG, which is the ferrimagnetic material of the MSW.
Weighting of the finger elements by changing the finger length has two distinct disadvantages in MSW devices. First, there is a reduced power transmission due to the shorter fingers at the edge of the transducer. Second, the short fingers excite different mode spectra than the long fingers. These modes are analogous to ordinary electromagnetic modes in waveguides, and, in MSW, occur in the ferrimagnetic material. These higher order modes cause unwanted passbands which severely limit the usefulness of such filters. For example, analogous tapers in antennas have on the order of a 20 dB amplitude range. For such a range in an MSW filter, very short fingers would be needed in the transducers. Providing such would cause strong coupling to higher order MSW modes, which is an undesirable situation particularly since typical MSW filters and MSW delay lines use overmoded ferrimagnetic samples or materials to maintain acceptable insertion loss.
Therefore, it is desirable, and it is an object of this invention, to provide the amplitude taper or weighting in the MSW transducers by an arrangement which does not require changing the dimensions or spacing of the individual finger elements of the transducers. It is another object of the invention to provide for phase reversals in transducer element circuits to further approximate Fourier transforms of desired filter passband characteristics.